Why are my cheeks losing fat continuously and why are my cheeks going very inside?

Fat loss in the cheeks tends to occur as people age but it can also be caused by other medical issues.  Losing weight due to diet or a medical problem, can cause the face to lose fat.  An evaluation by a doctor would be needed to try and determine the cause of your facial fat loss.  If there is no treatable medical condition causing the problem, there are a few treatment options.  Injectable fillers or surgically placed implants can help in appropriate patients.  

Volume and contouring of the cheeks can be created with well-placed fillers (Voluma, Radiesse, Bellafill, Contour) and Sculptra. Implants or fat are alternatives. Botox and radiofrequency can help slim and contour the jawline. Liposuction can help slim the face. Ultherapy, J Plasma and rf treatments can provide skin tightening and lift. I recommend getting a formal evaluation with a facial sculpting expert. Best, Dr. Emer

Volume loss can happen for several reasons including: aging and weight loss. Both implants and fillers in the cheeks can create good results for patients.  Both can build up the cheeks or bring symmetry to the face.  You must balance the pros and cons of each procedure and do what you feel is the best decision for you.
Fillers:
Pros: No downtime Reversible Quick
Cons: Temporary More expensive in long run
Implants Pros: Permanent Great for patients with poor bone structure Less expensive overtime
Negative: Risks associated with surgery Possible infection Downtime
I hope this helps!